Saw ladder filter with inter-stage matching saw resonator

Abstract

A SAW filter has SAW resonators coupled in a ladder network with at least one π-type section. Besides having a series-arm SAW resonator and two shunt-art SAW resonators, the π-type section has an inter-stage matching SAW resonator coupled in series with the series-arm SAW resonator. The inter-stage matching SAW resonator has an interdigital transducer with an electrode finger pitch substantially equal to the pitch in the series-arm SAW resonator. The inter-stage matching SAW resonator reduces impedance mismatching between the left half and the right half of the π-type section.

What is claimed is: 
     
       1. A resonator-type SAW filter having a plurality of SAW resonators with respective interdigital transducers formed on a single piezoelectric substrate, said SAW resonators being coupled in a ladder network configuration having at least one π-type section with a first ladder stage and a second ladder stage, said π-type section being formed by a series-arm SAW resonator and two shunt-arm SAW resonators, said π-type section also comprising: an inter-stage matching SAW resonator coupled in series with said series-arm SAW resonator, said inter-stage matching SAW resonator also having an interdigital transducer, the interdigital transducer of said inter-stage matching SAW resonator and the interdigital transducer of said series-arm SAW resonator having substantially equal electrode finger pitches, said inter-stage matching SAW resonator reducing impedance mismatching between the first ladder stage and the second ladder stage of said π-type section.   
     
     
       2. The resonator-type SAW filter of claim 1, wherein the interdigital transducer of said inter-stage matching SAW resonator and the interdigital transducer of said series-arm SAW resonator have exactly equal electrode finger pitches. 
     
     
       3. The resonator-type SAW filter of claim 2, wherein said inter-stage matching SAW resonator and said series-arm SAW resonator are combined into a single SAW resonator. 
     
     
       4. A resonator-type SAW filter having a plurality of SAW resonators with respective interdigital transducers formed on a single piezoelectric substrate, said SAW resonators being coupled in a ladder network configuration having at least one π-type section with a first ladder stage and a second ladder stage, said π-type section being formed by a series-arm SAW resonator and two shunt-arm SAW reasontors, said sereis-arm SAW ersonator having a first resonant frequency and a first antiresonant frequency, the interdigital transducer in said series-arm SAW resonator having a first electrode finger pitch, said π-type section also comprising: an inter-stage matching SAW resonator coupled in series with said series-arm SAW resonator, said inter-stage matching SAW resonator having a second resonant frequency and a second antiresonant frequency, said inter-stage matching SAW resonator having an interdigital transducer with a second electrode finger itch, said second electrode finger pitch being more narrow than said first electrode finger pitch, said second resonant frequency being higher than said first resonant frequency by a first amount, and second antiresonant frequency being higher than said first antiresonant frequency by a second amount substantially equal to said first amount, said inter-stage matching SAW resonator reducing impedance mismatching between the first ladder stage and the second ladder stage of said π-type section.   
     
     
       5. The resonator-type SAW filter of claim 4, wherein said first amount and said second amount are both greater than one megahertz but less than one hundred megahertz. 
     
     
       6. The resonator-type SAW filter of claim 4, wherein said π-type section has a plurality of inter-stage matching SAW resonators as described in claim 4, the interdigital transducers of said plurality of inter-stage matching SAW resonators having different electrode finger pitches, all more narrow than said first electrode finger pitch, said different electrode finger pitches causing the resonant frequencies of said plurality of inter-stage matching SAW resonators to be higher than said first resonant frequency by different third amounts and the antiresonant frequencies of said plurality of inter-stage matching SAW resonators to be higher than said first antiresonant frequency by different fourth amounts substantially equal to respective third amounts. 
     
     
       7. The resonator-type SAW filter of claim 6, wherein said third amounts and said fourth amounts are all greater than one megahertz but less than one hundred megahertz. 
     
     
       8. An improved SAW filter of the type that includes a piezoelectric substrate and a set of interdigital transducers on the substrate to provide a corresponding set of SAW resonators, the SAW resonators being connected in a ladder network configuration which includes a first ladder stage and a second ladder stage that is coupled to the first ladder stage to form a π-type section, the set of SAW resonators including a first shunt-arm resonator that is disposed in the first ladder stage, a second shunt-arm resonator that is disposed in the second ladder stage, and at least one series-arm resonator that is connected between the first and second shunt-arm resonators, wherein the improvement comprises: means provided by the at least one series-arm resonators for reducing impedance mismatching between the first ladder stage and the second ladder stage of the π-type section.   
     
     
       9. The SAW filter of claim 8, wherein the SAW filter is a bandpass filter. 
     
     
       10. The SAW filter of claim 8, wherein the at least one series-arm resonator comprises a plurality of the SAW resonators in the set, so that a pluralty of sereis-arm resonators are connected between the first and second shunt-arm resonators. 
     
     
       11. The SAW filter of claim 10, wherein all of the series-arm resonators that are connected between the first and second shunt-arm resonators have the same electrode finger pitches. 
     
     
       12. The SAW filter of claim 10, wherein one of the series-arm resonators that are connected between the first and second shunt-arm resonators has an electrode finger pitch that is different from the electrode finger pitch of another of the series-arm resonators that are connected between the first and second shunt-arm resonators.